U.S. Pat. No. 4,073,349 discloses a pneumatically operated sander which is driven by a rotary pneumatic motor via an eccentric connection with a gear train. The eccentric connection comprises an eccentric pin and a link. A shoe is driven by the link connected to the eccentric pin. The eccentric pin is connected to the gear train. In order to balance the dynamic forces generated by the sander element, a weight may be properly positioned within the tool housing and driven by a similar eccentric connection to a gear train. Thus the weight is driven reciprocatively in the opposite direction to the shoe. Such a construction has many elements. It is difficult to assemble and is very heavy.